1. Field of the Invention
The present invention relates to a receiver and a receiving method suitable for OFDM applied to mobile communication systems, wireless LAN systems and the like.
2. Description of the Related Art
Digital television broadcasting has been started recently. OFDM (Orthogonal Frequency Division Multiplexing) modulation is used as a transmission scheme for terrestrial television broadcasting systems in Europe and Japan. An advantage of the OFDM is its ability to reduce delay interference in multipath transmission channels which is essential transmitting conditions to terrestrial television broadcasting systems by transmitting wideband signals by a large number of orthogonal carriers.
Digital broadcasting in Japan has adopted ISDB-T. In ISDB-T, signal processing, such as error correction coding, interleaving coding, and digital modulation, is applied to a transport stream (TS) specified in the MPEG-2 standard, and the TS is OFDM-modulated and output.
In ISDB-T, 108 OFDM symbol carriers in the frequency domain make up one block and, one, two, or four blocks make up one segment, depending on modes. That is, the number of carries in one segment is 108, 216, or 432. In ISDB-T, a bandwidth equivalent to 13 segments is used for transmission.
Also in ISDB-T, 204 OFDM symbols make up one frame in the time domain. TS transmission and energy spreading are performed on a frame-by-frame basis.
ISDB-T allows hierarchical transmission in which multiple layers of data with different transmission properties are transmitted simultaneously. Each layer includes one or more OFDM segments, and parameters such as a carrier modulation scheme, a coding rate of inner codes, and a time interleave length can be specified for each layer.
Information about such layers and a frame synchronization signal is transmitted as TMCC (Transmission and Multiplexing Configuration Control). TMCC is into inserted every block of each symbol. That is, one carrier of each block is allocated to TMCC and one symbol contains 1-bit BPSK-modulated TMCC information. Accordingly, 204 bits of TMCC information is transmitted in one frame in each block.
16 bits of the 204 bits of TMCC information in an OFDM frame are a synchronization signal. A receiver detects the synchronization signal to establish frame synchronization. In ISDB-T, energy spreading is performed in order to prevent concentration of energy on a particular frequency. Energy spreading is accomplished by adding pseudorandom numbers to data by using a PN (pseudorandom number) generator. The initial value of the pseudorandom generation is provided at the start of a frame. Decoding cannot be performed unless a pseudorandom number value is known. Therefore, when a synchronization signal is detected halfway in a frame, synchronization is established at the start of the frame subsequent to the frame in which the synchronization signal is detected.
Thus, ISDB-T has a drawback that it takes a relatively long time to establish synchronization.
Therefore, Japanese Patent Laid-Open No. 2002-84255 discloses a technique that, when a synchronization signal is detected halfway in a frame, a pseudorandom number generated at the point is calculated, thereby enabling decoding to be started with the symbol immediately after the synchronization is detected. However, digital broadcasting, in which multiple modes and layers are set as mentioned above, requires a huge amount of calculation and memory capacity in order to calculate pseudorandom number values for all settings.
In ISDB-T, two types of synchronization signals having patterns different with each other are alternately used in every other frame. Accordingly, time for two or more frames is required for establishing full synchronization. Japanese Patent Laid-Open No. 2005-65219 discloses a technique that calculates reliability in order to successfully establish synchronization using only one frame. However, the technique proposed in Japanese Patent Laid-Open No. 2005-65219 cannot establish synchronization until the start of a next frame at which the initial value for energy spreading is provided even when a synchronization signal is detected halfway in the previous frame.